Gas chromatography paired with mass spectrometry (GC-MS) can be utilized in environmental analysis to extract samples of interest (which can contain impurities within rich chemical matrices) from media, such as food, soil or water. In an example, samples are separated in time utilizing gas chromatography (GC) and injected into an ionization source for compound ionization and identification using a mass spectrometric (MS) analysis.
Some GC-MS processes employ an electron ionization (EI) ion source to ionize the samples or compounds using an electron bombardment process to thereby produce a fragment spectra. Compounds are identified by comparing the generated spectra with a library of standard EI spectra. This technique can be used to identify up to one hundred compounds per run within a dynamic range of between low picograms (pg) to tens of nanograms (ng).
Two-dimensional gas chromatography (GCxGC) can broaden the identification to thousands of analyzed compounds per run but the EI spectra may not provide sufficient molecular peak statistics for a wide range of particularly fragile and volatile analytes. This can affect and contaminate proper identification.
In general, relatively softer ionization methods, such as chemical ionization (CI) and field ionization (FI), may be used to provide desired molecular peak information. CI may employ ion molecular reactions of a proton transfer and is highly selective (e.g., this provides strong suppression and interference for compounds with low proton affinity.) The CI source, however, can be poorly compatible with fast gas chromatography separation and is incompatible with two-dimensional gas chromatography having 10-20 ms peaks. FI is fairly universal but can be complicated, unstable, and insensitive with a typical detection limit of 100 pg (i.e., two orders lower compared to electron ionization.)
Photo ionization (PI) is another soft ionization method that has been used in connection with moderately polar compounds. In one instance, sealed UV lamps are used to ionize a GC eluent and ion current is thereafter measured, or ion compounds are identified using optical spectroscopy. It has been suggested to implement PI at atmospheric conditions for GC-MS analysis. In an example, PI is additionally accompanied by a damping of internal energy at atmospheric pressure, as such can make it softer as compared to vacuum UV ionization. Dopant vapors of acetone or benzene may be added to reinforce efficiency. Confusion in spectra interpretation can result, however, due to the resultant generation of ions, such as M+ ions, MH+ ions, ionic clusters and fragment ions. Moreover, there is a much higher spread of compound dependent ionization efficiency in PI compared to EI.
Glow discharge (GD) ionization methods have also been used. Direction ionization with glow discharge at 1-10 mbar gas pressure has been employed, but organic compounds exhibit significant fragmentation comparable to the fragmentation that occurs using an EI source thereby limiting the detection limit to about 1 picogram. Even when gas pressures are increased notable fragmentation is still observed. Separation of GD and reaction regions at atmospheric gas pressure diminishes efficiency and results in non-uniform ionization across a wide range of compounds, such as polar and non-polar organic compounds.
In short, analytical measurements of generic GC-MS are unsatisfactory and there remains a need for improved ionization methods that result in uniform efficiency of ionization across a wide range of polar and non-polar compounds.